Heterocycle Reactivity

Pyridine:
- The lone pair of electrons on the nitrogen atom is located in an $sp^2$ orbital.
- This $sp^2$ orbital is perpendicular to the π plane.
- The lone pair is available for bonding, making pyridine basic and nucleophilic.
Pyrrole:
- The lone pair of electrons on the nitrogen atom resides in a p orbital.
- This p orbital is in resonance with the other π electrons within the ring.
- As a result, pyrrole is not basic.

Due to its electron-poor nature (similar to an imine), electrophilic aromatic substitution on pyridine is generally unfavorable.
Electrophiles tend to add to the nitrogen atom.
Nitrogen lowers the orbital energy in pyridine’s π system compared to benzene, making electrophilic attack on the ring difficult.
When an electrophile attacks at C2 or C4, the positive charge is distributed on N, C3, and C5, which is energetically unfavorable due to the positive charge on nitrogen.
Electrophilic attack at C3 is slightly more favorable but still challenging because the positive charge is distributed on C2, C4, and C6.

Nucleophilic substitution reactions are facile on pyridine, similar to SNAr reactions.

Five-membered heteroaromatic rings, such as pyrrole, are well-suited for electrophilic aromatic substitution due to their electron-rich nature (similar to an enamine).

Indole is formed by the fusion of a benzene ring to a pyrrole.
It has a pyrrole-type nitrogen atom.
Indole features a $10\pi$ aromatic system and is electron-rich.
Electrophilic attack is more favorable at the C3 position because attack at C2 dearomatizes the benzene ring, which requires more energy as it disrupts the aromatic stability of benzene.

Tautomerization: Imidazole and pyrazole can undergo tautomerization due to the presence of multiple nitrogen atoms.
Enhanced Nucleophilicity and Basicity: Imidazole exhibits enhanced nucleophilicity and basicity.
Slightly Enhanced N–H Acidity: These compounds show slightly enhanced N–H acidity.

Electrophilic reagents (E+) tend to react with electron-rich aromatics.
Nucleophilic reagents (Nu–) tend to react with electron-poor aromatics (this is a simplification).
Electron-rich heterocycles + E+ → likely reaction
Electron-poor heterocycles + E+ → unlikely reaction
Electron-rich heterocycles + Nu– → unlikely reaction
Electron-poor heterocycles + Nu– → likely reaction
Electron-donating groups (EDG) may facilitate reactions of electron-poor heterocycles with electrophiles.
Electron-withdrawing groups (EWG) may facilitate reactions of electron-rich heterocycles with nucleophiles.

Pyridine, being electron-poor, can be activated to undergo electrophilic aromatic substitution.

5-membered heterocycles react with electrophiles.
In a molecule with multiple possible reaction sites, the C2 position is generally more reactive, but do not confuse with C3 reactivity of indole.

Leaving groups (LG) that are ortho or para to electron-withdrawing groups (EWG) are activated towards SNAr reactions.
Common leaving groups include halides, but -N2+, -NO2, and -SO3R are also good.
Among halides, the leaving group ability follows the order: F– > Cl– > Br– > I–.
EWG activates 5-membered heterocycles towards SNAr.
Turning lousy leaving groups into good ones by using 2-pyridone. OH is not a great leaving group but it can be converted by reaction with $(CF<em>3SO</em>2)<em>2O$ . Similarly, $POCl</em>3$ can be used.

Driven by complexation with a heteroatom.
Commonly ortho-directed.
Effectively, a deprotonation reaction using a strong base (n-BuLi, sec-BuLi, t-BuLi).
TMEDA is sometimes used to aid complexation.